Digital printing machines can take on a variety of configurations. One common process is that of electrostatographic printing, which is carried out by exposing a light image of an original document to a uniformly charged photoreceptive member to discharge selected areas. A charged developing material is deposited to develop a visible image. The developing material is transferred to a medium sheet (paper) and heat fixed.
Another common process is that of direct to paper ink jet printing systems. In ink jet printing, tiny droplets of ink are sprayed onto the paper in a controlled manner to form the image. Other processes are well known to those skilled in the art.
The primary output product for a typical digital printing system is a printed copy substrate such as a sheet of paper bearing printed information in a specified format. Quite often, customer requirements necessitate that this output product be configured in various specialized arrangements ranging from stacks of collated loose printed sheets, to brief reports stapled together, to tabulated and bound booklets. The sheets of media, usually paper, are compiled, stapled, and ejected at the last stage of the job, in a region called a finisher.
Various external output devices have been designed for connection to a digital printing machine. The paper will exit the printing system and be passed to an external finishing device, wherein a critical parameter in such delivery is the capability to operate at process speed so as to not inhibit the function of the printing machine.
Finishing procedures, such as sorting, collating, stapling and ejecting, require the movement of mechanical components. In state-of-the-art digital printing machines, it is common to have a quantity of sets in a job stream which require various sorts of finishing activities. In order to accommodate multiple sets, each set in the stream is typically held or delayed until the finishing activity of the preceding set has been completed. Moreover, it is often necessary to slow the output speed of the printing machine so as not to exceed the rate at which the external device, or finisher, can receive and process sets of output documents for producing the final output product. These finishing delay times detract from the overall productivity of the printing system.
Sheet buffering can be defined as holding sheets of paper within a finisher paper path while functions like compiling, stapling, and ejecting sets are accomplished. One type of finisher will skip a sheet in between each set in order to free up time to accomplish these functions. The problem with this method is that it slows productivity. Another finisher uses a system that compiles three sets at a time to buffer. A three tray set compiling unit fills as sheets enter the finisher. They are unloaded by an expensive clamping system that brings sets to the stapler. The problem with this system is excessive hardware and associated cost. Still another finisher uses buffering arms to temporarily hold sheets and then drop them into a compiler. The apparatus is costly, and further problems arise with registration issues and timing constraints which limit the use of this system. Yet another finisher uses a wait station to buffer a sheet. However, with higher speed finishing devices, this type of buffering does not work. An example of such a high speed finishing device is a newly introduced production finisher which operates at 157 ppm production rate.
An example of a sheet buffering system can be found in U.S. Pat. No. 5,303,017, filed on May 7, 1993, entitled, “Print Skip Avoidance For On-Line Compiling,” and assigned to the assignee hereof. An example of a sheet timing system can be found in U.S. Pat. No. 7,706,704, filed on Jun. 12, 2006, entitled, “Digital Printing Apparatus Having Substantially Equal Output Rates For Various Sheet Sizes And Orientations,” and assigned to the assignee hereof. An example of a sheet registration system can be found in U.S. Pat. No. 8,109,506, filed on May 29, 2009, entitled, “Sheet Observer With A Limited Number Of Sheet Sensors,” and assigned to the assignee hereof.
Accordingly, there is a need to provide a sheet buffering system that will vary the time interval of selected sheets in a finisher to allow finishing of a set.
There is a further need to provide a sheet buffering system of the type described and that not slow down the production rate of the printer.
There is a yet further need to provide a sheet buffering system of the type described and that is mechanically simple and robust, thereby minimizing cost and avoiding the problems associated with the prior art.